The invention concerns generally the receiving and decoding of digital signals over a radio interface. Especially the invention concerns the receiving and decoding of a signal where consecutive parts may have been modulated with a different modulation method in the transmitting end, when the receiver does not know the applied modulation method before receiving the signal.
Attempts have been made to enhance the throughput of digital radio transmission systems by allowing the use of different modulation methods according to the signal propagation conditions and/or the nature of the information to be transmitted. As an example we will consider the proposed EDGE cellular radio system (Enhanced Data rates for GSM Evolution), which is a further developed addition to the known GSM (Global System for Mobile telecommunications). EDGE recognizes two modulation methods, of which eight-level phase shift keying or 8-PSK allows for the representation of a sequence of three bits with one transmission symbol, whereas gaussian minimum shift keying or GMSK only uses one bit to create each transmission symbol. In each of these modulation methods the information content of a transmission symbol is coded into the phase angle of the symbol compared to a certain reference phase. The present invention is not limited to application with phase modulation methods, although some of its features require a number of obvious modifications if some other methods like amplitude or frequency based modulation are used.
FIG. 1 is a simplified representation of an EDGE transmitter-receiver pair. The transmitter comprises a channel encoder 101 to perform some channel coding, a modulator 102 to transform the stream of channel encoded bits into a corresponding stream of modulation symbols, and a phase rotator 103 to implement a selected phase rotation to the symbols in the stream produced by modulator 102. The operation of all these blocks is controlled by a control block 104 in a manner known as such. For example, the control block 104 possesses the information about which modulation mathod should be used at any given time, and what amount of channel coding implemented with which encoding method should be performed. A transmitter radio frequency part 105 converts the complete stream of phase rotated modulation symbols into a radio frequency signal that will be transmitted to the receiver.
The receiver comprises a receiver radio frequency part 110 to convert the received radio frequency signal to a lower frequency, a symbol derotator 111 to remove the phase rotation, a demodulator 112 to convert the stream of modulation symbols back to a sequence of bits, and a channel decoder 113 to remove the channel coding. The operation of these blocks is again controlled by a control block 114 that should be able to select the correct phase derotation, demodulation and decoding operations at any given moment.
In general terms the phase rotation may be construed to be a part of the modulation process and correspondingly the phase derotation may be construed to be a part of the demodulation process. In EDGE they are usually presented as separate operations, because the modulation and demodulation proper take place according to the known 8-PSK and QMSK principles, which are described in the literature in their masic form without additional phase rotations and derotations.
The transmission between the transmitter and the receiver takes place in bursts, and changing the modulation method is only allowed between bursts; a single burst is always modulated with a single modulation method in EDGE. Changing the modulation method in the middle of a burst would be technically possible but it would require somewhat complicated transmitter and receiver structures. In practical systems there are also limitations that require a certain minimum number of bursts to be transmitted with a certain modulation method before the modulation method is again changed. A widely proposed minimum number of consecutive bursts for this purpose is four.
The EDGE receiver generally does not know beforehand about the coming changes in the applied modulation method. Each burst comprises a so-called training sequence the constant form of which is known. At the phase derotation and demodulation stage the receiver checks, was the training sequence reproduced in its correct form. The different phase rotation methods associated with the use of different demodulation methods should ensure that only the correct demodulation method and its associated phase derotation produce the correct training sequence.
A problem arises when the signal propagation conditions at the radio interface are so bad that the receiver is not able to correctly recognize the applied modulation method on the basis of the above-explained phase rotation arrangement. Such bad conditions could mean that the received carrier-to-interference ratio (C/I) or bit energy per noise density (Eb/NO) is low, the delay spread in the received signal is long relative to the equalizer span of the receiver, or the relative speed between the transmitter and the receiver is high. The receiver usually derotates and demodulates the received bursts in their order of reception, and a decision once made can not be reversed even if it were subsequently found out that a certain burst or a number of bursts were demodulated with the wrong demodulation method.
An incorrectly demodulated burst will have an effect on the channel decoding stage. Channel encoding and decoding is usually performed on blocks of information that are longer than the contents of one burst. A typical block to be channel encoded and decoded houses the contents of four bursts. If one or several of these bursts were demodulated with the wrong demodulation method, the decoding of the whole block will probably fail causing a retransmission request to be transmitted from the receiver to the transmitter.
There are two obvious solutions to the above-mentioned problems. The first solution would be to explicitly indicate to the receiver the modulation method that was used to modulate each burst. The indication should be conveyed to the receiver with very high reliability, because an incorrectly received indication would only make the problem worse. This solution would increase the signalling needs between the transmitter and the receiver, which is an undesired direction of development. The second solution is to have a large memory in the receiver and to store each received burst in its undemodulated form long enough for the receiver to be sure which method should be applied in its demodulation. The is solution is uneconomical since the demodulation and decoding operations are already quite memory-intensive and the solution would again considerably increase the amount of memory to be built into the receiver.
It is an object of the present invention to provide a method and a receiver arrangement that reduce the detrimental effects of incorrectly recognized modulation in a receiver. It is an object of the invention that it should not considerably increase the signalling needs between the transmitter and the receiver or the amount of required memory in the receiver.
The objects of the invention are achieved by using soft decoding at the channel decoding stage and by suppressing the influence of the incorrectly demodulated symbols through the use of neutral soft decoding values for their representation. The incorrectly demodulated bursts are most advantageously recognized by using the known limitations of allowed modulation changes.
The method according to the invention is characterized in that
soft decoding is used to decode a block of digital information, wherein each subblock of said block is converted to a sequence of soft decoding values associated with certain probabilities of allowed state transitions in the decoding process,
for each subblock it is determined, after its demodulation, whether the correct demodulation method was selected for it and
a subblock for which it is found that the correct demodulation method was not selected is converted to a sequence of neutral soft decoding values indicating equal probabilities for all allowed state transitions in the decoding process.
The invention also applies to a receiver which is characterized in that
the decoder contained therein is a soft decoder arranged to handle each subblock as a sequence of soft decoding values associated with certain probabilities of allowed state transitions in the decoding process,
the receiver is arranged to determine for each subblock whether the correct demodulation method was selected for it and
the decoder is arranged to convert such subblocks to sequences of neutral soft decoding values for which it is found that the correct demodulation method was not selected, where said soft decoding values indicate equal probabilities for all allowed state transitions in the decoding process.
If conventional channel decoding arrangements are used, incorrectly demodulating a burst will produce a sequence of bits which represent basically a random selection of bit values 0 and 1. However, there is also known a concept called soft decoding, which means that the demodulator does not give unambiguous bit values at its output but a number of transition probabilities that represent the probability of certain state transitions in the channel decoder. By using soft decoding (as in some cases of more general digital decoding) it is possible to correctly decode even a digital information sequence where some bits or bit combinations have completely unknown values. According to the invention, after it has been established that a certain burst has been incorrectly demodulated, the corresponding demodulation results are replaced with a sequence of soft decoding values that give an equal probability to all allowed state transitions in the channel decoder.
Interleaving is usually applied when the contents of a channel encoded block of information is mapped into transmission bursts. This ensures that even if one burst is lost in the receiver in the sense that it only gives rise to a sequence of neutral soft decoding values, it may still be possible to correctly decode the block of information to which the burst belonged: the neutral soft decoding values will only appear at isolated locations in the sequence that is input to the channel decoder, so the information about allowed state transitions in the channel decoder and the correctly demodulated soft decoding values from the other burst(s) may be enough to reconstruct the original bit sequence that was channel encoded in the transmitter. The probability of correct channel decoding despite of xe2x80x9cerasuresxe2x80x9d increases if the receiver has the possibility of using iterative decoding, where the results of a previous decoding round as well as the original demodulated soft decoding values are used as input information to a subsequent decoding round.
An advantageous way of recognizing a certain burst as incorrectly demodulated is to observe the received burst sequence in the light of the known limitations that govern the allowed changes of modulation method. If at a certain point it is found out with reasonable certainty that a change of modulation method has occurred, it may immediately be deduced that at least as many bursts as the predefined minimum number before the observed change should have been demodulated with the first demdulation method, and at least as many bursts after the observed change must be demodulated with the second demodulation method.